In an article recently published in Nucleic Acids Research, Luigi Martino, Maria Conte (Kings’ College, London) and collaborators report a new RNA binding mode for the human La protein to an internal region of the hepatitis C virus mRNA (Martino L. et al. (2011)) .
This work emphasises the high plasticity of the La autoantigen which can accommodate profoundly different RNA baits, and sheds new light on a protein which definitely has more diverse cellular functions than just controlling metabolism of 3’UUUOH ending nuclear transcripts.
To know more….
Human La protein (hLa) is an essential player in the biology of both coding and non-coding RNAs. Within the nucleus, La associates with all newly synthesised RNA polymerase (pol) III transcripts, as well as a subset of pol II small nuclear and nucleolar RNA intermediates, by binding specifically to the common (U)n-OH trailer present at the 3´ termini of these RNAs. This interaction protects the 3´-end of the target RNA from exonucleolytic cleavage and, in doing so, favours retention in the nucleus for proper endonucleolytic processing and folding. These activities have been observed in eukaryotes as diverse as yeast and humans and map to a highly conserved region within the N-terminal half of the human protein that contains the La motif (LaM), an elaborated winged-helix domain, appended to an RNA recognition motif, RRM1. The LaM and RRM1 domains operate as a single RNA binding unit (recently re-named the ‘La module’) for the specific recognition of 3´- UUUOH single-stranded RNA. In the cytoplasm however human La has been shown to interact with an array of different mRNAs lacking a 3´-UUUOH trailer, including internal ribosome entry sites (IRES) found in a subset of cellular mRNAs or in the positive-sense RNA genomes of viruses such as poliovirus, human immunodeficiency virus (HIV) or hepatitis C virus (HCV). In these cases, the hLa-IRES interaction appears to augment translation, although the molecular mechanism remains obscure, in part because the details of the hLa-RNA interaction have not been elucidated.
In this article, through a systematic biophysical investigation, we revealed that hLa binds to domain IV of the HCV IRES using an RNA recognition mechanism that is quite distinct from its mode of binding to RNAs with a 3´-UUUOH trailer: although the La motif and the RRM1 that together comprise the N-terminal La module are sufficient for high-affinity binding to 3´-UUUOH, recognition of HCV domain IV requires the La module to work in concert with the atypical RRM2, which had not previously been shown to have a significant role in RNA binding. This new mode of binding does not appear strongly sequence-specific, but recognises structural features of the RNA, in particular a double-stranded stem flanked by single-stranded extensions. Our results provide new insights into the remarkable adaptability of the hLa protein in RNA recognition and assign a clear role to the C-terminal region of hLa in HCV IRES domain IV recognition, building on past functional studies to provide a solid framework for investigation of how La contributes to IRES-directed translation initiation. Moreover, the essential features of the alternative mode of RNA recognition unveiled here may hold true for other La proteins given their resemblance across higher eukaryotes, though will not be shared by yeast homologues that do not harbour an RRM2. It appears therefore that some of the key RNA binding activities of La are a prerogative of higher eukaryotes.
Maria R. Conte (King’s College, London)
and to know even more… (Martino L. et al. (2011))
Very nice work!!